The initiating event in the pathogenesis of atherosclerosis and restenosis following angioplasty is injury to cells in the artery wall1. Injury or stress stimulates signalling and transcriptional pathways in vascular smooth muscle cells, stimulating their migration and proliferation and the eventual formation of a neointima. Smooth muscle cell proliferation is a key feature of neointima formation, atherosclerosis, restenosis and graft failure.
c-Jun, a prototypical member of the basic region-leucine zipper protein family, is transiently induced following arterial injury in animal models2,3. c-Jun forms both homodimers and heterodimers with other bZIP proteins to form the AP-1 transcription factor. While investigations over the last decade have linked AP-1 with proliferation, tumorigenesis and apoptosis, AP-1 has also been implicated in tumor suppression and cell differentiation4. Thus, gene-targeting strategies that down-regulate c-Jun expression do not necessarily inhibit cell proliferation.
Kanatani et al, (1996)5 have shown that antisense oligonucleotides targeting c-Jun dose-dependently reduce the growth-inhibitory effect of dexamethasone and TGFβ. Recent reports indicate that c-Jun NH2 terminal kinase/stress activated protein kinase (JNK), an upstream activator of c-Jun and numerous other transcription factors, is expressed by SMCs in human and rabbit atherosclerotic plaques6,7 and that dominant negative JNK inhibits neointima formation after balloon injury8. c-Jun, however, has not been localised in human atherosclerotic lesions, nor has it been shown to play a functional role in arterial repair after injury.
It is clear, however, that the finding that c-Jun, or any other given gene, is inducibly expressed in the artery wall following balloon angioplasty does not necessarily translate to it playing a positive regulatory role in transcription, proliferation or neointima formation. For example, it has been shown that three transcriptional repressors (NAB2, GCF2, and YY1) are activated in vascular smooth muscle cells by mechanical injury in vitro, as well as in the rat artery wall. NAB2 directly binds the zinc finger transcription factor Egr-1 and represses Egr-1-mediated transcription9. GCF2 is a potent repressor of the expression of PDGF-A, a well-established mitogen for vascular smooth muscle cells, and inhibits smooth muscle cell proliferation10. Similarly, YY1 overexpression blocks smooth muscle cell growth without affecting endothelial cell proliferation11.
c-Jun can repress, as well as activate transcription. c-Jun binds the corepressor TG-interacting factor (TGIF) to suppress Smad2 transcriptional activity12. c-Jun also blocks transforming growth factor beta-mediated transcription by repressing the transcriptional activity of Smad313.
c-Jun can inhibit, as well as stimulate proliferation. Using antisense oligonucleotides to c-Jun, Kanatani and colleagues demonstrated that inhibition of human monocytoid leukemia cell growth by TGF-beta and dexamethasone is mediated by enhanced c-Jun expression5.
c-Jun, however, has not been directly linked to the complex process of angiogenesis, which underlies many common human diseases including solid tumor growth and corneal disease. Angiogenesis is a complex multi-step process involving proteolytic degradation of the basement membrane and surrounding extracellular matrix, microvascular endothelial cell proliferation, migration, tube formation and structural re-organisation14.